This invention relates to testing apparatus for measuring thermal properties of a liquid quenchant, particularly to apparatus and process steps for monitoring the cooling properties of polymer quenchants, and apparatus and process steps for monitoring the coolant properties of used liquid quenchants and altering their properties, e.g., restoring the cooling properties of the used liquid quenchants to their original, unused condition.
Thermally-responsive elements, known as thermistors, consist of semi-conductors having a negative temperature coefficient of resistance, i.e., have the characteristics of decreasing resistance with the increasing temperature. The thermistors are hard, ceramic-like, semi-conductors. They are available in at least three distinct forms; beads, discs or washers, and rods. All of these types are made of various mixtures of the oxides of magnesium, nickel, cobalt, copper, uranium, iron, zinc, titanium, and manganese. The mixtures of oxides are formed into the desired shapes and sintered under accurately controlled atmospheric and temperature conditions. They are characterized by being small and compact in size, are highly stable, are mechanically rugged and shock resistant, are provide with permanent electrical contacts, have a wide range of resistance to temperature coefficient and power dissipation, and have substantially unlimited life when operated within their maximum temperature rating. Typically, they are designed to have a specific negative coefficient of resistance, i.e., their resistance will change from several thousand ohms at 25.degree. C. to near zero at 500.degree. C.
Thermistors are known to be useful for detecting various properties of fluids. For example, thermistors are often positioned in gas and liquid streams to detect changes in thermal conductivity as it relates to flow rate or flow stoppage. For example, see U.S. Pat. No. 3,236,099, in which a bridge circuit including two thermistors is used in apparatus for indicating material stream flow characteristics by calorimetry. In most cases, the thermistor circuit is simple used to activate an alarm or control function.
Thermistors are often used as temperature sensing probes in resistance thermometers. For example, see U.S. Pat. Nos. 2,876,327; 3,699,813 and 4,143,549, which utilize thermistors in conjunction with Wheatstone bridge circuits. In this particular application, they are subjected to external heating, but in other applications they may be subjected to internal heating by the application of an appropriate electric potential. When voltage is applied to a thermistor and resistor in series a current will flow in the circuit. This electrical current causes heat to be generated in the thermistor, which in turn causes the resistance of the thermistor to be lessened and permits more current to flow than if the resistance had remained constant. This process continues until the thermistor reaches the maximum temperature possible for the amount of power available in the circuit, at which time a steady state will exist, i.e., the electrical energy applied is equal to the heat energy given off by the thermistor and resistor. Of course, the medium surrounding the thermistor will affect the equilibrium temperature.
U.S. Pat. No. 4,364,677 discloses apparatus comprising a bridge circuit which includes two thermoresistant devices such as thermistors which are used in heated probes for comparing the thermal conductivity of a gemstone to that of a standard stone such as a diamond. Other systems for thermal testing of solids are disclosed in U.S. Pat. Nos. 4,488,821 and 3,457,770. For example, the steady state temperature attained in air will be significantly higher than that for a liquid. Even minor differences in the thermal conductivity of liquids will have an effect.
The application of thermistors in devices used to measure the mixture ratio of a two-component gas mixture is known, (see, e.g., U.S. Pat. No. 3,683,671) but no such use with liquid mixtures has been found in the prior art. In particular, no reference to the use of thermistor circuits for determination of used liquid quenchant quality has been found in the prior art, nor is use of a thermistor known for testing used quenchants to determine the amount of quenchant additives to be added to the used quenchant to bring its quenching properties to the desired level.
U.S. Pat. No. 2,937,334 disclosed heat transfer testing apparatus for evaluating the heat transfer capacity of materials including quenching media, comprising a heated metal probe and electronic circuitry including a bridge circuit for determining the occurrence of the Curie point as the metal probe cools. The use of a thermistor is neither disclosed nor suggested. U.S. Pat. No. 3,333,470 discloses a method and apparatus for sensing fluid properties, including a resistance-type temperature sensor and a bridge circuit for maintaining at constant temperature. The system measures heat transfer between the sensor and its environment, which information can be used in the measurement of temperatures, velocities, concentrations and like properties of fluids. The use of a thermistor as the sensor is not suggested, and a method of determining a fluid's concentration is not indicated.
Heating and subsequent cooling is a process used to change the physical properties of various metals, such as steel and aluminum alloys. The overall cooling rate as well as the cooling rate at intermediate temperatures is critical in the quenching process, as these quantities, in combination with the composition of the metal, control to a great degree the final structural properties of the heat treated part. Many different liquids have been employed as quenchants over the years, including water, various oils, and even human blood.
Aqueous solutions of synthetic organis polymers have gained wide acceptance as quenching media in the last two decades. These products not only provide cooling characteristics intermediate between the fast quenching action of water and the relatively slow action of oil, but also provide a high degree of flexibility in that the desired cooling rate is set by adjustment of the polymer to water ratio. The most commonly used polymer quenchants are polyalkylene glycols, polyvinvylpyrrolidones and polyacrylates, with the polyalkylene glycol materials being by far the most common.
When a water quenchant loses its quenching properties due to contamination or an oil quenchant becomes contaminated or degraded, the quenchant is generally replace, purified or reconditioned. However, to insure the satisfactory operation of polymer-containing quenchants, the bath temperature, degree of agitation and optimum polymer concentration must be established and maintained. This is most often done by refractive index and kinematic viscosity measurement. The polymer concentration is most often determined by the end user through the use of a hand held refractometer. Kinematic viscosity measurement is used as an alternate method when the presence of bath contaminants such as inorganic salts affect the accuracy of the refractometer. However, after prolonged use or under unusually severe operating conditions, a significant increase in the water-soluble contaminants present and/or a molecular weight change in the polymer due to degradation may alter the bath to a point that the original viscosity-quenching action relationship is no longer valid.
These measuring techniques, moreover, relate only to fresh polymer solutions and are affected by the presence of soluble contaminants and the eventaul degradation of the polymer. When this condition occurs the quenching bath is typically checked by cooling curve analysis. This a a rather cumbersome laboratory procedure that entails the use of a temperature instrumented metal probes that are heated and immersed in baths of fresh and used quenchant solutions for the development of comparative cooling rate values.
It would be desirable to have available a portable cooling curve apparatus for use in the field, but this approach is not particularly practical from the standpoint of unit cost and operational complexity. However, a simple heat transfer (thermal conductivity) test designed to simulate fluid quenching action of a liquid quenchant would have merit.